tilde.chat online status badges

thanks to shields.io and custom badge support, we now have user count badges!

ExampleURLResult
Network user counthttps://tilde.chat/badges/badge.phpuser count badge
User count for #metahttps://tilde.chat/badges/badge.php?channel=%23meta#meta badge
Invalid channel name (error handling)https://tilde.chat/badges/badge.php?channel=invalidinvalid badge